GC-15: Implemented functionality for including extra DNS names in server certificates:

- Added functional test covering the new scenario.
- Updated invocations of relevant commands in existing code to pass-in
the list of extra DNS names where appropriate.
- Updated server command and high-level function for issuing server
certificates to accept list of additional DNS subject alternative
names to include in certificate.
- Fixed existing unit tests.
- Added additional unit tests that cover the new function.